高能球磨结合粉末冶金法制备碳纳米管增强7055Al复合材料的微观组织和力学性能
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Microstructure and Mechanical Properties of Carbon Nanotubes-Reinforced 7055Al Composites Fabricated by High-Energy Ball Milling and Powder Metallurgy Processing
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通讯作者: 刘振宇,zyliu@imr.ac.cn,主要从事纳米碳增强金属基复合材料相关研究
收稿日期: 2020-07-06 修回日期: 2020-09-28 网络出版日期: 2021-01-27
基金资助: |
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Corresponding authors: LIU Zhenyu, associate professor, Tel:
Received: 2020-07-06 Revised: 2020-09-28 Online: 2021-01-27
作者简介 About authors
毕胜,男,1990年生,博士生
采用高能球磨结合粉末冶金工艺制备了碳纳米管(CNT)含量(体积分数)分别为0、1%和3%的CNT/7055Al复合材料。采用OM、SEM、TEM以及拉伸实验等方法研究了CNT/7055Al复合材料的CNT分布、晶粒结构、近界面结构及力学性能,分析了复合材料的强化机制和各向异性。结果表明,CNT/7055Al复合材料为无CNT的粗晶区与富集CNT的超细晶区组成的双模态晶粒结构;CNT在Al基体的超细晶区中分散良好,CNT-Al界面干净清洁,界面反应产物少;3%CNT/7055Al复合材料沿挤压方向的抗拉强度达到816 MPa,但延伸率仅为0.5%。细晶强化和Orowan强化是CNT/7055Al复合材料主要的强化机制。由于CNT沿不同方向的增强效率不同以及粗晶条带组织的存在,复合材料表现出比基体合金更强烈的各向异性,在垂直挤压方向的拉伸性能要弱于沿挤压方向的拉伸性能。
关键词:
In the recent years, lightweight and high-strength structural materials have gained much attention in engineering applications. Carbon nanotube (CNT)-reinforced Al (CNT/Al) composites are promising structural materials owing to the good mechanical properties and high reinforcing efficiency of CNTs. Previous studies on these composites mainly focused on fabricating CNT-reinforced low-strength or medium-high-strength Al alloys (such as pure Al, or 2xxx series or 6xxx series Al alloys) composites via various dispersion methods. However, only few studies investigated composites with super-high-strength Al alloys as the matrices. In the present work, CNT/7055Al composites with CNT volume fractions of 0%, 1%, and 3% were prepared by high-energy ball milling combined with powder metallurgy. The CNT distribution, grain structure, interface, and mechanical properties of the CNT/7055Al composite were investigated using OM, SEM, TEM, and tensile tests. The strengthening mechanism and anisotropy of the composite were analyzed. The results indicated that the composite had a bimodal grain structure consisting of CNT-free coarse grain zones and CNT-enriched ultrafine grain zones. CNTs were well dispersed in the ultrafine grain zones of the Al matrix, and the CNT/Al interface was clean. There were only few reaction products at the interface. The tensile strength of the 3%CNT/7055Al composite reached 816 MPa, but the elongation was only 0.5%. Grain refinement and Orowan strengthening were the main strengthening mechanisms of the CNT/7055Al composite. Because of the load transfer efficiency of CNTs and a coarse grain band structure, the composite exhibited stronger anisotropy than the matrix alloy. The tensile properties of the CNT/7055Al composite normal to the extrusion direction were weaker than those in the extrusion direction.
Keywords:
本文引用格式
毕胜, 李泽琛, 孙海霞, 宋保永, 刘振宇, 肖伯律, 马宗义.
BI Sheng, LI Zechen, SUN Haixia, SONG Baoyong, LIU Zhenyu, XIAO Bolv, MA Zongyi.
非连续增强铝基复合材料因其高比强度、高比刚度、良好耐磨性和尺寸稳定性在航空、航天等领域有广阔的应用前景[1~6]。与陶瓷颗粒相比,纳米增强相尤其是碳纳米管(CNT)具有超高强度(约30 GPa)、超高模量(约1 TPa)和低的热膨胀系数(约0 K-1),被认为是铝基复合材料理想的增强体[7~10]。然而由于大的长径比和Van der Waals力作用,CNT在Al基体中难分散、易团聚,而团聚态CNT无法充分发挥强化效果[11,12]。在过去十几年里,研究人员尝试了多种化学和机械方法以实现CNT在Al中均匀分散,如片状粉末冶金法、原位自生法、分子级别混合法、湿混法和搅拌摩擦加工法等[13~18]。其中,属于机械分散方法的高能球磨法工艺简单,可以批量生产复合材料粉末,是近年来研究学者广泛采用的制备CNT增强铝基复合材料(CNT/Al)的方法[19~21]。
高能球磨法通过磨球对粉末的高速剪切作用实现CNT分散[22],在分散CNT的同时,CNT结构也受到一定程度的损伤。近年来研究人员多对球磨工艺进行探索。如Liu等[23] 研究了球磨时间对CNT/Al复合材料性能的影响,发现随着球磨时间延长,CNT逐渐分散,但CNT损伤也不断增加,过长球磨时间导致复合材料力学性能变差。Choi等[24]研究了球磨转速对CNT分布及损伤的影响,发现在低球磨转速下,CNT位于Al粉表面上容易受到损伤。在中等球磨转速下,由于CNT嵌入Al粉内部,CNT损伤减小。Liu等[22]通过建立数学模型分析了不同球磨转速下粉末片状化时间对CNT分散的影响,并得到实验结果的验证。Xu 等[19]对现有的球磨工艺进行优化,提出变速球磨工艺,在保证CNT分散的同时,降低了CNT损伤。
然而,目前大部分的研究主要是关于CNT在低强度或者中高强度铝合金(如纯Al、2xxx系或者6xxx系铝合金)中的分散制备研究[25~28],所报道的复合材料强度普遍不高。为获得高性能CNT/Al复合材料,有必要对CNT增强超高强度的7xxx系铝合金展开研究,目前仅有极少数相关报道。如Xu等[29]通过元素合金化和片状粉末冶金工艺制备出CNT低损伤且分布均匀的CNT/7075Al复合材料,其韧性相比传统工艺制备复合材料有较大改善,但其强度相比超高强铝合金优势还不够明显。在现有报道的关于CNT/Al复合材料的研究中,仍有部分关键问题未深入涉及。如研究人员主要针对CNT形貌、分布、CNT-Al界面等进行表征[24,30],但对基体的微观结构表征并不详细,比如晶粒结构[27,31],尤其是高合金元素含量的超高强铝合金基体的晶粒结构鲜有文献报道,其对CNT/7xxx Al复合材料的性能有何影响还缺乏理解。此外,在复合材料作为结构件时,其受力往往是多方向的,有必要探索材料的各向异性。然而,目前在力学性能方面,研究人员多对沿CNT分布方向进行测试,而对于垂直CNT分布方向的性能报道则较少[8]。因此,本工作以超高强7055Al合金为基体[32],采用高效的高能球磨工艺分散CNT,结合粉末冶金方法制备复合材料,并对复合材料的微观组织、强化机制及各向异性展开深入分析,以期为制备高强度CNT/Al复合材料提供依据。
1 实验方法
采用7055Al合金球形粉末作为原料,合金成分(质量分数)为Al-8.1%Zn-2.2%Mg-2.2%Cu,纯度99.9%,平均直径为10 μm,如图1a所示。实验所用CNT为多壁CNT,纯度为97.5%,直径为10~15 nm,长度为2~5 μm,如图1b所示。分别将体积分数为1%和3%的CNT与7055Al合金粉末放入1-S搅拌式球磨机中,加入1.6% (质量分数)的硬脂酸作为过程控制剂阻止粉末冷焊,粉末总质量为1 kg,球磨机转速为400 r/min,球料比为15∶1,球磨时间为6 h,获得CNT/7055Al复合材料粉末。为方便对比,采用相同工艺对7055Al合金粉末进行了球磨。将CNT/7055Al复合材料和7055Al粉末在500℃保温1.5 h进行热压,随后将热压锭在420℃以17∶1的挤压比挤压成棒材。将挤压棒在470℃做固溶处理,保温1 h后快速水淬,随后立即在120℃时效24 h (T6处理)。
图1
图1
7055Al合金球形粉末和碳纳米管(CNTs)的SEM像
Fig.1
SEM images of the as received 7055Al alloy powders (a) and carbon nanotubes (CNTs) (b)
采用DMi8M金相显微镜(OM)观察复合材料微观组织。采用SUPRA 55和QUANTA 600扫描电子显微镜(SEM)观察复合材料断口形貌。采用Tecnai G2 20透射电子显微镜(TEM)观察复合材料晶粒结构及CNT的微观分散情况。采用Jobin Yvon HR800 Raman光谱仪评估CNT损伤及界面反应程度,其中激光波长为532 nm,光谱范围为600~2000 cm-1。采用D8 Advance X射线衍射仪(XRD)分析复合材料的位错密度。在进行XRD射线衍射扫描前,对样品进行电解抛光以消除样品表面残余应力,电解液配比为30 mL HF+11 g H3BO3+970 mL H2O。复合材料腐蚀液选用Graff Seagent试剂,配比为1 mL HF+16 mL HNO3+3 g CrO3+83 mL H2O。
T6态复合材料中的位错密度通过XRD谱中Al峰的宽化数据测得,首先采用Williamson-Hall法对Al峰进行拟合[33]:
式中,B为真实峰宽,λ为Cu靶的激发波长(0.154 nm),ε为微应变,θB为Bragg衍射角,D为晶粒尺寸。将BcosθB与sinθB绘制于同一坐标系中进行线性拟合,通过拟合直线的斜率和截距得到ε和D。再将ε和D代入到
式中,b为Burgers矢量模,对于Al基体来说其值约为0.286 nm。
使用Instron 5982拉伸试验机对沿挤压方向的T6态样品进行拉伸测试,应变速率为1×10-3 s-1,试样为标距15 mm、直径3 mm的棒状试样。使用Instron 5848拉伸试验机对垂直于挤压方向的T6态样品进行拉伸测试,应变速率为1×10-3 s-1,试样为标距5 mm、厚度为1.2 mm的板状试样。每种复合材料至少取3个测试样品,并取平均值。
2 实验结果
2.1 复合材料微观组织
图2a和b为T6态CNT/7055Al复合材料腐蚀后的OM像。可以看出,复合材料由深色和浅色2种组织构成,初步推断深色区域为细晶组织,浅色区域为粗晶组织,这种结构与Liu等[27]制备的双模态CNT/2009Al复合材料的组织相类似。CNT/7055Al复合材料中的粗晶组织呈条带状,长度为数十微米,宽度约为1 μm,并沿挤压方向定向分布。这与文献报道的CNT/纯Al[11,19,35]、CNT/2xxxAl[36]等复合材料中较均匀的晶粒组织有明显不同。复合材料中粗晶结构的产生可能有2种原因,第1种原因是部分晶粒未受增强粒子的钉扎,在材料热压过程中发生长大。如Li等[31]在低温球磨TiB2/5083Al复合材料中也观察到过类似的实验现象,认为是部分晶粒未受TiB2粒子钉扎长大导致;第2种原因是7xxx系铝合金的合金元素更多,固相线温度低,复合材料在热压过程中出现了少量瞬态液相,在冷却过程中形成粗晶组织。
图2
图2
T6态CNT/7055Al复合材料OM像和背散射SEM像
Fig.2
OM (a, b) and back-scattered SEM (c, d) images of T6 treated 1%CNT/7055Al (a, c) and 3%CNT/7055Al (b, d) composites (The longitudinal direction is the extrusion direction)
图3
图3
T6态1%CNT/7055Al复合材料晶粒结构和CNT分布的TEM像
Fig.3
TEM images of grain structure (a) and CNT distributions (b) in T6 treated 1%CNT/7055Al composites (The black and white arrows denote CNT and extrusion directions, respectively)
图4
图4
T6态CNT/7055Al复合材料细晶区晶粒的TEM像
Fig.4
TEM images of grain structures in fine grain zones of T6 treated 1%CNT/7055Al (a) and 3%CNT/7055Al (b) composites
2.2 复合材料析出相形貌
图5
图5
T6态1%CNT/7055Al复合材料中粗晶内和细晶内析出相的TEM像
Fig.5
TEM images of the precipitates of T6 treated 1%CNT/7055Al composites in a coarse grain (a) and in a fine grain (b) (The arrows denote precipitates)
2.3 损伤与界面
一般采用Raman光谱中D峰(约1350 cm-1)和G峰(约1570 cm-1)强度的比值(ID/IG)评估CNT的损伤。D峰代表石墨层中的缺陷,而G峰代表石墨层的结晶度[19,47]。ID/IG越大,表示CNT的损伤越大。图6为3%CNT/7055Al复合材料球磨6 h后的粉末、T6态挤压棒和CNT的Raman光谱。可以看出,对于球磨后复合材料粉末和热处理后的复合材料块体,其G峰向大的Raman频移方向移动,由约1570 cm-1偏移到1610 cm-1处,这主要与CNT受到压缩应变有关[30]。CNT沿轴向的热膨胀系数接近0,而Al的热膨胀系数为23.6×10-6 K-1,这使得复合材料粉末或块体在冷却过程中Al的收缩要远大于CNT,导致CNT受到压应变。经过6 h的球磨,复合材料粉末的ID/IG由1.00增加到1.18,说明球磨工艺对CNT的结构造成了一定的损伤。在T6态复合材料的Raman光谱中,Raman频移为约490 cm-1和约860 cm-1处出现Al4C3的特征峰。通过Al4C3的特征峰的强度,可以判断出样品中Al4C3的含量并不高[29,48]。这表明,虽然CNT在球磨时遭受一定损伤,但在热压过程中并没有发生严重的界面反应。与Xu等[19]通过高能球磨法制备的CNT/Al复合材料相比,本工作中由界面反应生成Al4C3的量更少。可以注意到,在TEM像(图3b)中仅观察到少量Al4C3,但在Raman光谱中却出现了明显的Al4C3峰,这是2种测试方法的特点所决定的。TEM是通过衬度来反映物相的,只有反应形核长大生成相,产生衬度差才能在TEM像中有所反映;而Raman光谱是通过晶格振动对激光的Raman散射效应来反映物相的,它对分子结构、结晶结构等信息敏感[47]。即使没有相的形核长大,而只是形成一定数量的化学键,在Raman光谱上即会有所反映。
图6
图6
3%CNT/7055Al复合材料球磨6 h粉末及T6态复合材料的归一化Raman光谱
Fig.6
Normalized Raman spectra of 3%CNT/7055Al powders milled for 6 h, T6 treated composites, and raw CNT (The D band and G band represent the presence of defects in graphite layers and the highly crystalline graphite, respectively; ID and IG represent corresponding peak intensities of D band and G band, respectively)
图7为T6态1%CNT/7055Al复合材料中CNT与Al基体近界面的HRTEM像。可以看到,CNT的管状结构保持完整,这是由于在球磨过程中CNT受到的是磨球剪切作用,CNT只在断裂的位置受到严重损伤,中间的管壁结构并没有被破坏。还可以看出,CNT与Al基体的界面干净,没有观察到界面产物的生成。CNT完好的管壁结构和干净的CNT-Al界面为获得高性能CNT/Al复合材料提供了基础。
图7
图7
T6态1%CNT/7055Al复合材料中CNT-Al近界面的HRTEM像
Fig.7
HRTEM image of interface of CNT and Al in T6 treated 1%CNT/7055Al composites
2.4 力学性能
表1为T6态CNT/7055Al复合材料沿挤压方向的力学性能。可以看出,随着CNT含量的增加,复合材料的抗拉强度增加,但同时延伸率下降。相比基体合金,1%和3% CNT的加入使复合材料抗拉强度分别提升了60和116 MPa,弹性模量也分别提升了8%和11%。图8对比了不同体系的CNT/Al复合材料的抗拉强度[8,13,14,16,19,22,28,29,49~51]。可以看出,以7系铝合金为基体的复合材料可以获得更高的强度。其中3%CNT/7055Al复合材料的抗拉强度达到816 MPa,但延伸率仅为0.5%,这归因于以下几个方面:(1) 球磨法制备的CNT长度普遍较短,数量多,弥散分布于基体,限制了基体晶粒的变形以及晶粒内部位错的运动;(2) 复合材料细晶区晶粒尺寸仅为200~300 nm,晶粒容纳位错能力有限,并且位错容易在晶界处湮灭,导致了复合材料变形能力较差。
表1 T6态CNT/7055Al复合材料的力学性能
Table 1
Volume fraction of CNT % | Yield strength MPa | Tensile strength MPa | Elongation % | Modulus GPa |
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0 | 657±11 | 700±8 | 2.8±0.6 | 73 |
l | 692±14 | 760±10 | 1.8±0.3 | 79 |
3 | 730±2 | 816±4 | 0.5±0.1 | 81 |
图8
图8
不同体系的CNT/Al复合材料的力学性能
Fig.8
Mechanical properties of CNT/Al composites with different matrices
表2为T6态7055Al和1%CNT/7055Al复合材料在不同拉伸方向的力学性能。可以看出,复合材料存在明显的各向异性,并且无论是7055Al合金还是复合材料在垂直于挤压方向的性能都要弱于沿挤压方向的性能。相比于7055Al合金,复合材料的抗拉强度在沿挤压方向增加60 MPa,但在垂直于挤压方向只增加18 MPa。说明CNT在垂直于挤压方向的增强效率大大降低。此外,7055Al合金在2个拉伸方向上的延伸率变化不大,而复合材料在垂直挤压方向上的延伸率相比平行于挤压方向上略有降低。
表2 T6态7055Al及1%CNT/7055Al复合材料在不同方向拉伸性能
Table 2
Sample | Orientation relationship between extrusion direction and tension direction | Yield strength MPa | Tensile strength MPa | Elongation % |
---|---|---|---|---|
7055Al | Parallel | 657±11 | 700±8 | 2.8±0.6 |
Perpendicular | 607±3 | 644±8 | 2.4±0.6 | |
1%CNT/7055Al | Parallel | 692±14 | 760±10 | 1.8±0.3 |
Perpendicular | 618±4 | 661±9 | 1.0±0.5 |
2.5 断口分析
图9
图9
拉伸方向平行于挤压方向和垂直挤压方向时T6态1%CNT/7055Al复合材料拉伸断口的SEM像
(a, b) tension direction parallel to extrusion direction(c, d) tension direction perpendicular to extrusion direction
Fig.9
Low (a, c) and high (b, d) magnified SEM images of fractographs of T6 treated 1%CNT/7055Al composites (The arrows in Figs.9b and d denote CNT)
3 分析讨论
3.1 Al与C的界面结合
式中,ΔG为Gibbs自由能,T为热力学温度。由
3.2 强化机制
一般来说,复合材料中的强化机制主要有:细晶强化、Orowan强化、位错强化和载荷传递强化[16],本工作对1%CNT/7055Al复合材料中的强化机制进行分析,3%CNT/7055Al复合材料的强化机制与其类似。
在机械球磨的过程中,复合材料晶粒得到大幅细化,从而使复合材料的基体强度明显提高,即产生了细晶强化。细晶强化可以通过Hall-Petch公式描述[55]:
Orowan强化是CNT/Al复合材料的另一个强化机制。位错的运动会受到复合材料中弥散质点的钉扎作用,从而导致强度增加。对于Orowan强化,本工作仅考虑了7系铝合金中常见析出相(η')的作用,忽略CNT的作用,因为大多数CNT分布在复合材料的晶界上[57]。由Orowan强化引起的强度变化(Δσorowan)可以通过下式来表达:
表3 析出相平均间距(λp)与平均半径(r)的测量结果
Table 3
Position | λp / nm | Precipitate length / nm | Precipitate width / nm | r / nm |
---|---|---|---|---|
In coarse grains | 26.30 | 17.90 | 2.90 | 5.20 |
In fine grains | 22.00 | 9.60 | 0.95 | 2.65 |
Mean | 24.15 | 3.93 |
此外,位错通过与自身的交互作用以及阻碍自身运动,也可以强化材料。位错密度越大,材料的屈服强度越大。由位错强化引起的强度变化(Δσdislocation)可以通过下式计算[56]:
式中,α为常数,对于fcc金属为0.2。由
本工作获得了较强的Al-C界面结合,因此,CNT的载荷传递作用得到充分发挥,载荷传递机制表达式如下[58]:
式中,σcy为复合材料的屈服强度;σmy为基体的屈服强度;Vf为增强相的体积分数;s为增强相的长径比,通过测量,s约为10。
综上所述,复合材料的屈服强度应是上述强化机制的总和,表达式如下:
3.3 复合材料拉伸各向异性的原因
CNT/7055Al复合材料在不同方向拉伸性能不同主要基于以下几个原因。首先,在不同方向上CNT的载荷传递效率不同。由载荷传递弱化引起的性能下降(Δσ)可以通过下式表示[8]:
式中,seff为有效长径比,θ为相对于CNT排列方向的离轴角。由于CNT多数沿挤压方向排列,在垂直于挤压方向,CNT的seff约为0,说明CNT在垂直于挤压方向几乎没有增强作用,图9d断口中的CNT形貌也验证了这一结果。经计算,1%CNT/7055Al复合材料由载荷传递引起的性能差异为18 MPa,但复合材料在2个方向的屈服强度相差为74 MPa,说明还有其它因素导致复合材料性能的下降。
4 结论
(1) CNT/7055Al复合材料为由细晶区和粗晶条带组成的双模态晶粒结构,粗晶条带沿挤压方向定向排列,均匀分布在细晶区中。CNT均匀分布在细晶组织中,且大多沿挤压方向排列;而在粗晶条带中观察不到CNT。
(2) CNT/7055Al复合材料中CNT结构完整性保持良好,CNT/Al界面干净、清洁,界面反应产物少。3%CNT/7055Al复合材料抗拉强度达到816 MPa。细晶强化和Orowan强化是CNT/7055Al复合材料的主要强化机制。
(3) CNT/7055Al复合材料存在比7055Al合金更严重的各向异性,在垂直挤压方向的拉伸性能要明显弱于沿挤压方向的拉伸性能,其主要原因是在垂直挤压方向CNT几乎不起载荷传递作用,以及复合材料中粗晶条带组织引起裂纹的快速扩展。
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